The present invention pertains to apparatus and methods for monitoring magnetic fields and further pertains to apparatus and methods for detecting the presence of ferromagnetic materials.
There has been need heretofore for improved instruments and methods for monitoring magnetic phenomena. It is often desirable to monitor changes in the configuration of the magnetic field generated by nuclear magnetic resonance ("NMR") medical imaging equipment and the like. Also, ferromagnetic materials can impair operation of medical NMR instruments and may cause a safety hazard in operation of such instruments. Accordingly, there have been needs heretofore for practical instruments and methods to detect ferromagnetic materials.
Devices known as Hall effect transducers have been used heretofore for monitoring magnetic fields. A Hall effect device provides an electrical signal which typically varies directly with the magnitude of the magnetic field component in one direction along a predetermined sensing axis. An oppositely directed field component along the same axis will produce an opposite or inverse signal. Magnetic field components transverse to the sensing axis typically do not produce a signal Certain commercial Hall effect devices have been provided with ferromagnetic flux directing elements. For example, one Hall effect transducer commercially available under the designation Model BH-850 High Sensitivity Hall Generator available from the F.W. Bell Company of Orlando, Fla., includes a Hall effect device in conjunction with an elongated ferromagnetic bar. The axis of elongation of the bar is aligned with the sensing direction of the transducer. The flux directing element tends to strengthen the response of the transducer.
The low voltage signals typically provided by Hall effect devices have been amplified heretofore to provide useful output signal levels. However, magnetic field monitoring instruments utilizing Hall effect devices heretofore have been subject to undesirable phenomena such as thermal drift or changes in response of the instrument with temperature. Also, many of the magnetic field sensing devices available heretofore have been incapable of detecting small changes in the orientation of the magnetic field.